Add Custom Formula Item Templates with AI generation and wizard integration

Introduces per-company reusable NCalc2 pricing formula templates for complex
fabricated items (roof curbs, enclosures, welded frames). Templates support
two output modes — FixedRate (formula yields a dollar amount) and SurfaceAreaSqFt
(formula yields sq ft fed into the standard coating engine). Includes:

- CustomItemTemplate entity, migration (AddCustomItemTemplates), IUnitOfWork repo
- IsCustomFormulaItem / CustomItemTemplateId / FormulaFieldValuesJson flags on
  QuoteItem, JobItem, CreateQuoteItemDto; mapped in all 3 JobItemAssemblyService
  overloads and all existingItemsData JSON projections + pageMeta blocks
- ICustomFormulaAiService / CustomFormulaAiService: Claude-powered formula
  generator (natural language + optional diagram image) and NCalc2 evaluator
- CompanySettings CRUD endpoints: GetCustomItemTemplates, Create/Update/Delete,
  UploadTemplateDiagram, TemplateDiagram (blob serve), EvaluateFormula, GenerateFormulaFromAi
- Company Settings "Custom Formulas" tab + cfModal + company-settings-custom-formulas.js
- item-wizard.js: formula item type card, renderFormulaFields, wzFormulaRecalc
  (live evaluate via POST), collectStep2 formula branch, buildCardHtml / emitHiddenFields
- Formula badge in Quotes/Details and Jobs/Details; AI badge gap fixed in Jobs/Details
- Help article (CustomFormulaTemplates.cshtml), Help Index card, HelpController action,
  HelpKnowledgeBase entry; 225/225 unit tests passing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Help/Index.cshtml

@@ -258,6 +258,22 @@
                     </div>
                 </div>
             </div>
+            <div class="col-md-6">
+                <div class="card border-0 shadow-sm h-100">
+                    <div class="card-body">
+                        <div class="d-flex align-items-start gap-3">
+                            <div class="rounded-3 bg-info bg-opacity-10 p-2 flex-shrink-0">
+                                <i class="bi bi-calculator text-info fs-4"></i>
+                            </div>
+                            <div>
+                                <h5 class="card-title mb-1">Custom Formula Item Templates</h5>
+                                <p class="card-text text-muted small mb-2">Build reusable NCalc pricing formulas for complex fabricated items like roof curbs, enclosures, and frames.</p>
+                                <a asp-controller="Help" asp-action="CustomFormulaTemplates" class="btn btn-sm btn-outline-info">Read more <i class="bi bi-arrow-right ms-1"></i></a>
+                            </div>
+                        </div>
+                    </div>
+                </div>
+            </div>
         </div>
     </div>